1. Field of the Invention
This invention relates generally to a knee joint prosthesis which replaces the articulating knee portion of the femur and tibia, and more particularly, to a floating bearing knee joint prosthesis having a fixed tibial post.
2. Discussion of the Related Art
A knee joint prosthesis typically comprises a femoral component and a tibial component. The femoral component and the tibial component are designed to be surgically attached to the distal end of the femur and the proximal end of the tibia, respectively. The femoral component is further designed to cooperate with the tibial component in simulating and articulating motion of an anatomical knee joint.
Motion of a natural knee is kinematically complex. During a relatively broad range of flexion and extension, the articular or bearing surfaces of a natural knee experience rotation, medial and lateral angulation, translation in the sagittal plane, rollback and sliding. Knee joint prostheses, in combination with ligaments and muscles, attempt to duplicate this natural knee motion, as well as absorb and control forces generated during the range of flexion. Depending on the degree of damage or deterioration of the knee tendons and ligaments, however, it may be necessary for a knee joint prosthesis to eliminate one or more of these motions in order to provide adequate stability.
While knee joint prostheses are effective in replacing the anatomical knee joint, they nevertheless follow different design options that each have different advantages and disadvantages. For example, many knee joint prostheses are polycentric, such that the posterior femoral radius is smaller than the distal femoral radius. The reason for the change in radius is to provide an appropriately sized femoral component, facilitate early rollback, and to allow for full flexion. However, because of the smaller posterior femoral radius, such knee joint prostheses provide a large articulating contact area in extension, but as the knee joint prosthesis moves during flexion, the smaller posterior radius creates a smaller articulating contact area as flexion increases. This generally reduces the contact area from a large surface contact area to a smaller contact area between the femoral component and the tibial component. For example, the contact area may be about 275 mm.sup.2 at extension and reduce substantially to about 75 mm.sup.2 at 60.degree. to 90.degree. of flexion. This reduced contact area may not be optimal for long term durability due to increased contact stresses.
Other knee joint prostheses attempt to eliminate this disadvantage by using a roller and trough articulation, where the roller and trough are spherically or cylinderally shaped to provide full contact area throughout the range of motion from extension to flexion. However, the disadvantage with most of these type knee joint prostheses is that posterior rollback of the femoral component relative to the tibial component is eliminated. Such rollback is extremely desirable because it increases the extension moment arm, increases quadriceps efficiency, and helps patients perform activities of daily living, such as climbing stairs. In addition, these types of designs are typically rotationally constrained.
Other knee joint prostheses attempt to eliminate the above disadvantages using another design option, such as a meniscus or floating bearing between the femoral component and the tibial component. By utilizing a floating bearing, the articulating contact area between the femoral component and the bearing can theoretically be increased without increasing constraint. However, some of the knee joint prostheses that provide a floating bearing rely on ligaments for femoral rollback which in many cases may not be reliable.
Knee joint prostheses are also generally provided having different levels of constraint. For example, primary type knee joint prostheses provide the least level of constraint, posterior stabilized (PS) knee joint prostheses provide an intermediate level of constraint, while fully constrained type knee joint prostheses provide the highest level of constraint upon the kinematic motions of a knee joint. In some situations, a patient may initially have a less constrained type knee joint implanted, such as a primary or posterior stabilized (PS) knee joint prosthesis. Should the patient later require a fully constrained type knee joint prosthesis because the patient is exhibiting instability, a surgeon is generally required to remove the entire knee joint prosthesis and implant both a new femoral and tibial component or a new fixed bearing to provide a fully constrained knee joint prosthesis. However, such extensive surgical modifications increases the overall surgical cost and complexity of upgrading a knee joint prosthesis from one constraint to another.
What is needed then is a knee joint prosthesis which does not suffer from the above mentioned disadvantages. This in turn, will provide a substantially conforming contact area between the articulating surfaces of the femoral component and the tibial component throughout extension and flexion, increase and provide a substantially constant articulating contact surface area from extension through flexion, reduce overall stresses and wear in the articulating contact area, provide femoral rollback relative to the tibial component by means of a floating bearing, provide a mechanical engagement mechanism to force femoral rollback without having to rely on soft tissue ligaments, increase the overall reliability of the knee joint prosthesis, prevent anterior movement of the bearing and provide a modular knee joint prosthesis that can achieve different levels of constraint by simply replacing a guide post that is fixed to the tibial component. It is, therefore, an object of the present invention to provide a floating or rotating bearing knee joint prosthesis with a fixed tibial post that achieves the above-identified advantages.